1. Field of the Invention
This invention relates to a method of transmitting an asynchronous transfer mode (ATM) cell over an ATM network between an ATM input interface and an ATM output interface.
2. Related Art
In a conventional ATM network, when a cell arrives at the ATM input interface, initial values are inserted into the virtual path identifier (VPI) and virtual channel identifier (VCI) fields in the header. The VPI field provides the coarse level routing for the ATM cell and the VCI field provides the fine level routing. At each switching node between the ATM input interface and the ATM output interface, the VPI and/or VCI fields are read and the output port is selected in accordance with the value of one or both of these fields using the routing table contained in the node. Before forwarding the cell on the selected output port, the values of one or both of these fields are updated.
In each switching node, the routing table and the updating values for the VPI and/or VCI fields are set up partly by network management and partly by signalling during call set up. Consequently, a conventional ATM network provides a connection-oriented service. In a connection-oriented service, the call is set up by signalling at the commencement of the cell and cleared down at the end of the call. Alternatively, the routing table and the updating values may be set up to provide a permanent connection.
If it is desired to transmit data over a switched ATM network, for example data from a computer provided with an ATM card, it is necessary to set up a call before the data can be transmitted. If the data is then transmitted more or less continuously for the duration of the call, the connection-oriented service provided by a conventional ATM network is satisfactory. However, it is often desired to transmit data intermittently, one burst of cells at a time followed by a long pause before the next burst of cells is transmitted. However, this incurs an overhead cost of setting up and clearing down the call for each burst of cells. Thus, a conventional switched ATM network is not optimal for intermittent transmission of data.
In order to provide a connectionless service over an ATM network, it has been proposed to add a server to provide a connectionless service as an overlay to an ATM network. However, this solution suffers from the disadvantage that there is still a call set up phase, in this case between the input interface and the server.